Recec , that doesn't really sound too hard, if you would like to bump up the difficulty?
To show off the strength of Mate's jaws, have him solo Chuck using only biting moves. Considering that you should only have Bite/Crunch & Ice Fang, and I'm pretty sure Poliwrath resists both those types, I'd say that is challengeing.
If it's not challenging enough (or perhaps too challenging) I can add/remove restrictions.
Mecha Ajnin
Take Sun Wukong the Pansage.
After offending the spirits and heaven and being trapped under a mountain for 500 years, the spirits decided the best way to discipline Sun Wukong was to send him on a pilgrimage to India with the monk, Tripitaka, unfortunately Sun Wukong mistook you for the monk, and is now travelling around Kalos with you instead.
Sun Wukong is capable of many great feats, one of which is to transform into many objects. Teach him Substitute when you get the TM. Sun Wukong is also a master at fighting so his three remaining moveslots must be damaging moves, so he has an attack ready for every situation. Upon fulfilling the Sub + 3 attacks condition he may finally evolve.
Sun Wukong also has a bit of a personality problem... in that despite being king of the monkeys, he's basically an immature child. Every time that he uses a super effective move his childish nature may get the better of him. Flip a coin, if heads, he may remain in battle, tails and he must switch (this represents his childish nature taking over, and the headband stopping him).
Sun Wukong must protect his team from Lysandre's Mega Gyrados. During this battle the coin flip rule doesn't apply. If Sun Wukong can beat Mega Gyrados without items or switching, then his pilgrimage has come to an end and he no longer needs to use the coin flip rule.
If he fails to solo Mega Gyrados, the coin flip rule still applies until the e4. Choose one e4 member, the coin flip rule doesn't apply for them, if he is able to solo them, the pilgrimage then ends and he no longer needs to flip coins fo the rest of the game... all 4 battles of it.
TL;DR
Pansage called Sun Wukong
Must know Substitute + 3 damage dealing moves
Can evolve when he knows Substitute + 3 attacks
When he uses a super effective move flip a coin
>Heads nothing happens
>Tails switch immediately
Solo Mega Gyrados without items.
>Coin flip rule does not apply against Mega Gyrados
>If he beats Mega Gyrados, the coinflip rule no longer applies.
>If he fails to solo Mega Gyrados, solo an e4 member of your choice, again without coinflips
>Soloing an e4 member also removes the coin flip rule
GL;HF